Cylindrical fiber probe devices and methods of making them

ABSTRACT

This invention involves a fiber probe device and a method of making it. The probe includes a relatively thick upper cylindrical region, typically in the form of a solid right circular cylinder, terminating in a tapered region that terminates in a relatively thin lower cylindrical region (tip), typically also in the form of a solid right circular cylinder, the lower region having a width (diameter) in the approximate range 0.01 μm to 150 μm.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of Marchman 2, applicationSer. No. 08/173,298 filed Dec. 22, 1993, now abandoned, which is acontinuation-in-part of Marchman 1, application Ser. No. 08/091,808,filed Jul. 15, 1993, now abandoned, both of which are incorporatedherein by reference.

TECHNICAL FIELD

This invention relates to probe devices, and more particularly tometrological fiber probe devices and to methods of making and usingthem.

BACKGROUND OF THE INVENTION

More than 100 years ago, the famous physicist Ernst Abbe described afundamental limitation of any microscope that relies on any lens orsystem of lenses in an imaging system to focus light or other radiation:diffraction obscures (makes fuzzy) those details of the image that aresmaller in size than approximately one-half the wavelength of theradiation. See "Scanned-Probe Microscopes" by H. Kumar Wickramasinghe,published in Scientific American, Vol. 261, No. 4, pp. 98-105(Oct.1989). In other words, the resolution of the microscope is limitedby the wavelength of the radiation. In order to circumvent thislimitation, researchers have investigated the use of, inter alia,various types of imaging probes. Scanning tunneling microscopy(hereinafter "STM" ) devices, atomic force microscopy (hereinafter"AFM") devices, and near-field scanning optical microscopy (hereinafter"NSOM") are some examples of different types of probe microscopes.

In STM, a metallic probe is brought sufficiently close to a conductingsample surface such that a small tunneling current is established. Themagnitude of this current is extremely dependent on the tip-to-sampledistance (i.e., topography of the sample surface). The tip is allowed toscan laterally across the (irregular) surface of the sample body withseveral angstroms separation between tip and sample in order to achieveimaging with atomic-scale resolution. The tunneling current, and hencethe tip-to-sample separation, is detected and controlled by anelectromechanical feedback servomechanism. In AFM, imaging is achievedin a similar manner to that of the STM except that the atomic forces(either short-range repulsive or long-range attractive) are detectedinstead of tunneling current. An obvious advantage to this technique isthat the tip and sample do not have to be conductive (all materialsexert atomic forces).

An NSOM device is typically comprised of an aperture located at the tipof an elongated optical probe, the aperture having a (largest) dimensionthat is smaller than approximately the wavelength of the opticalradiation that is being used. During device operation, the probe ispositioned in close proximity to the surface of a sample body. Theaperture of the probe is then allowed to scan across the surface of thesample body at distances of separation therefrom all of which distancesare characterized by mutually equal forces components exerted on theprobe device in the direction perpendicular to the global (overall)surface of the sample body, the scanning being detected and controlledby an electromechanical feedback servomechanism as was the case in STMand AFM.

For example, U.S. Pat. No. 4,604,520, describes, inter alia, a probedevice having an aperture located at the tip of a cladded glass fiberthat has been coated with a metallic layer. The aperture is drilled intothe metallic layer at the tip of the fiber at a location that is coaxedwith the fiber. The (immediate) neighborhood of the tip is composed of asection of solid glass fiber that has obliquely sloping (truncatedconical) sidewalls, whereby the sidewalls do not form a cylinder of anykind. Therefore, as the probe device laterally scans a rough surface,the calculations required to determine the desired information on theactual contours (actual profile) of the surface of the sample bodyrequire prior detailed knowledge of the slanting contours of thesidewalls of the probe, and these calculations typically do not yieldaccurate metrological determinations of the desired profile of thecontours of the surface of the sample body, especially at locations ofthe surface of the sample body where sudden jumps (vertical jumps)thereof are located. In addition, fabrication of the probe device iscomplex and expensive, especially because of the need for drilling theaperture coaxially with the fiber.

Another example involves the fabrication of nanometric tip diameterfiber probes for photon tunneling microscopes "PSTM" ) by selectivechemical etching of the GeO₂ -doped cores of optical fibers. See"Reproducible Fabrication Technique of Nanometric Tip Diameter FiberProbe for Photon Scanning Tunneling Microscope", Togar Pangaribuan, etal., Japan Journal Applied Physics, Vol. 31 (1992), pp. L 1302-L 1304.By selectively etching the GeO₂ doped regions of the fiber, a taperedtip having the shape of a small cone can be formed on the endface of theoptical fiber. The cone angle of the fiber probe tip is controlled byvarying the doping ratio of the fiber core and the composition of theetching solution. A fiber probe with a cone angle of 20° and tipdiameter of less than 10 nm was fabricated. Only probes havingconical-shaped endfaces can be made with this technique, so that thesidewalls do not form a cylinder of any kind. The scanning range of sucha probe is undesirably limited owing to the relatively large width(diameter) of the endface on which the relatively short-width conicaltip is centered, coupled with the fact that, during scanning, the probeis rastered from side -to-side in an arc: a desired large length of scanis attempted, the corners of the probe's endface undesirably will makecontact with the sample surface. In addition, the conical shape of thetip undesirably limits the accuracy of measurements wherever the surfacebeing probed has a sudden jump.

SUMMARY OF THE INVENTION

This invention involves, in a specific device embodiment, a probedevice, that can be used for surface metrology (for probing andmeasuring contours of surfaces) as an STM, AFM, or NSOM device,comprising a fiber having a relatively thick upper cylindrical regionterminating in a tapered region that terminates in a relatively thinright cylindrical lower region, i.e., with straight vertical sidewalls.The lower cylindrical region advantageously has a maximum width in theapproximate range of 0.05 μm to 150 μm, and it terminates at its bottomextremity in an essentially planar end surface oriented perpendicular tothe axis of the thin right cylindrical region. As used herein, the term"maximum width" refers to the maximum diameter--i.e., the length of thelongest line segment that can be drawn in a cross section of acylindrical region of a fiber segment, the line segment being orientedperpendicular to the axis of the cylinder, from one extremity of thecross section to another. In the case of a circular cylindrical region,the width (=diameter) in any direction of each such cross section isthus equal to this maximum width. Also, as used herein the term"approximate" has its ordinary meaning in accordance with significantfigures.

The invention also involves methods of making such a probe device andthen using it for surface metrological purposes such as STM, AFM, orNSOM--i.e., making the probe device and moving it across the surface ofa sample body whose contours are to be measured.

The fact that the lower region of the probe device terminate in a planarend surface-advantageously oriented perpendicular to the axis of thecylinder-enables accurate positioning and hence position-determinationsof the probe at locations of a surface of the sample body being scannedby the probe, even at sudden jumps in the surface. And the fact that thelower region of the probe device has the form of a cylinder simplifiesthe determination of the profile of the surface of the sample body. Theprobe's sidewalls advantageously are coated with a suitable layer, suchas an optically reflecting layer, for confinement of the light insidethe fiber probe tip especially if and when the probe is used for probingas an NSOM device--i.e., if and when the probe device is moved with itstip (lowest extremity) maintained at successive distances from a surfaceof a sample body where the forces exerted by the sample body on theprobe device are mutually equal.

BRIEF DESCRIPTION OF THE DRAWING

FIGS. 1-4 depict elevational cross-section diagrams of a probe devicewith straight vertical sidewalls being fabricated in accordance with aspecific embodiment of the invention; and

FIGS. 5 and 6 depict elevational and horizontal cross-section diagrams,respectively, of a probe device in an early stage if its fabrication inaccordance with another specific embodiment of the invention.

Only for the sake of clarity, none of the FIGURES are drawn to anyscale.

DETAILED DESCRIPTION

Referring to FIG. 1, a fiber segment 10, typically an optical fibersegment, typically takes the form of a solid right circular cylinder. Atop portion of the cylindrical sidewall surface of this segment 10 iscoated with a polymer resist layer 20 that is resistant to hydrofluoricacid etching, whereby a lower portion (having a predetermined height H)is not coated. The glass fiber segment 10 has a bottom endface 11 thatis fiat and is oriented in a plane perpendicular to the axis of the(cylindrical) segment 10. The segment 10 is firmly attached to a holder40, typically made of teflon, by a thin segment 31 of suitable materialcoated with an adhesive, such as a segment of adhesive tape.

Advantageously, the polymer resist layer 20 is a chlorofluorocarbonpolymer dissolved in an organic solvent typically comprising a ketone oran ester or a mixture of a ketone and an ester. For example, the polymerresist is a copolymer formed by polymerizing vinylidene fluoride andchlorotrifluoroethylene commercially available as a resin from 3MCorporation under the tradename "KELF" Brand 800 Resin, which isdissolved in amyl acetate or other suitable organic solvent to theextent of approximately 30-to-50 wt percent resin.

The material of the fiber segment 10 can be, but need not be, uniform.For example, it can have a central core surrounded by a peripheralcladding as known in the art of optical fibers. At any rate, thematerial of the fiber segment 10 typically is circularly symmetric. Thefiber segment 10 is immersed (FIGS. 1 and 2) in a wet essentiallyisotropic etch, typically a buffered oxide etching solution 50-such as asolution composed of 2 parts (7:1) buffered oxide etch, 1 parthydrofluoric acid, 1 part acetic acid, and 1 part H₂ O. As used herein,the term "essentially isotropic etching" refers to those cases in whichthe etching rates in the axial and radial directions do not differ fromeach other by more than approximately 10 per cent.

The acetic and H₂ O components of the solution 50 help dissolve theaccumulation of unwanted residual material on the fiber surface duringetching. The etching solution 50 is contained in a container 60, and ithas a level 51 that intersects the polymer layer 20 at a predeterminedheight, whereby the entire (lower) portion of the surface of the fibersegment 10 that is not coated with the polymer layer 20 is submerged inthe solution 50.

After the fiber segment 10 has been immersed in the etching solution 50for a predetermined amount of time, it assumes the shape shown in FIG.2--that is, a relatively thick upper region 23, in the form of a solidright circular cylinder, terminating in tapered intermediate(transition) region 22, in the form of a tapered solid circular region,and terminating in a relatively thin lower cylindrical region 21, in theform of another solid right circular cylinder.

The purpose of the polymer layer 20 is to create an etching boundarythat is spaced from the air-to-etching-solution interface whereby ashorter undercut portion 20 enhances the overall stability of the probe.

For example, the height (length) H (FIG. 1) of the bottom portion of thefiber segment 10, which is not coated with the polymer resist layer 20,is typically equal to approximately 2.5 cm; and the diameter D (FIG. 1)of the fiber segment 10 is typically equal to approximately 125 μm ormore. After having been etched with the solution 50, the thin lowerregion 21 has a diameter 2R (FIG. 2) typically equal to approximately 30μm or more, as determined by the duration of the immersion.

Next, the bottom face of this lower region 21 is cleaved in a planeoriented perpendicular to the (common) axes of the upper region 23 andthe lower region 21, as by means of a fiber cleaver aided by opticalmicroscopic viewing or other micrometer controlled procedure. In thisway, the height of the lower cylindrical region 24 becomes reduced to apredetermined value h (FIG. 3), and the tip thereof is a planar surfaceoriented perpendicular to the axis of this lower cylindrical region 24.This height h is typically in the approximate range of 0.05 μm to 30.0μm, advantageously in the approximate range of 1 μm to 10 μm. Thepolymer layer 20 is then removed ("stripped" ), or it can be removedprior to the cleaving. More specifically, some (or all) of the polymerresist layer 20 is removed such as by immersion in acetone, whereby aresidual polymer resist layer 25 remains (or not).

The fiber segment again is immersed (FIG. 4) in the essentiallyisotropic etching solution 50, for another predetermined amount of time,to a solution level 52 that intersects the segment at a level locatedtypically above the top of the tapered region 22 and that isotropicallyetches those portions of the fiber with which it comes in contact. Inthis way another intermediate solid cylindrical region 44 formed; andthe resulting lower region 41 of the fiber segment is still a solidright circular cylinder but having a reduced diameter equal to w, butthe height h thereof is not reduced by a significant amount. That is tosay, the height h remains essentially unchanged. At the same time, thediameters of the various portions of the resulting tapered intermediateregion 42 of the fiber segment are reduced. At the location of thesolution level 52, a meniscus of the etching solution 50 produces agradual tapered transition between regions of the fiber immediatelyabove and immediately below the solution level 52, as indicated in FIG.4.

The etching solution level 52 (FIG. 4) optionally can be adjusted to bethe same as, or to be slightly below, the top of the tapered,intermediate region 22. In such a case, there will be no intermediatecylindrical region 44. The lower region 41, intermediate region 42, theintermediate region 44 (if any), and upper region 43 thus all take theform of mutually coaxial solid cylinders, typically circular cylinders.The diameter w of the lower portion 41--i.e., the width of the tip ofthe resulting probe (FIG. 4)--can be adjusted to any desired value byadjusting the amount of time during which the immersion in the solution50 is allowed to continue. This width w can be made in the approximaterange of 0.01 μm and 150 μm, typically approximately 0.05 μm to 0.5 μm,and advantageously in the approximate range of 0.05 μm to 0.2m-depending on the ultimately desired metrological use of the probe whenmeasuring sample surfaces, i.e., depending on the desired metrologicalresolution of the measurements to be made by the fiber during itssubsequent use as a probe device.

Typically, such metrological use involves scanning the surface of asample body with the probe while holding the probe with anelectromechanical feedback servo-mechanism, as known in the art, all ofwhich distances are characterized by mutually equal components of force(for the case of AFM) in the direction perpendicular to the overallsurface of the sample body.

The predetermined time durations of the immersions for the etchings(FIGS. 1-2 and 4) can be determined by trial and error.

FIGS. 5 and 6 depict an alternative manner by which the fiber segment 10can be held. Specifically, the fiber segment 10 is not coated with thepolymer resist layer 20, and it is held in place by a segment ofadhesive tape 31 that is affixed to a holder 45. This holder 45 can bethe same as the holder 40 previously described. In such a case,immersion of the fiber segment in the solution 50 causes a meniscus toform between the solution 50 and the fiber segment 10, whereby the lowerregion 21 and the intermediate region 22 are defined.

The above-described method of probe device fabrication does not rely ona doping profile of the fiber segment 10 or the composition of theetching solution 50 (FIG. 4). The cylindrical tip is formed by reducingthe diameter of the fiber primarily by the second etching (FIG. 4) inthe radial direction. The flat end face results from the cleaving step(FIG. 3). Throughout the second etching step (FIG. 4), the lower region41 maintains its shape, even as its dimensions (in all directions)decrease. Any structures created on the bottom endface, such as a coneor point, are eliminated as the diameter w becomes smaller thanapproximately 1 μm. For large diameters, the formation of features onthe end face is prevented by the cleaving step (FIG. 3) because notenough etching occurs in the axial direction to produce a significantvariation in the amount of etching at different radii of the fiber. If alarge amount of etching in the axial direction is desired, a flatendface can still be achieved by using a fiber segment with uniformdoping profile or (even in the presence of a radial gradient of dopingconcentration in the fiber segment) by properly adjusting theconcentration of the chemical components of the etching solution, or bydoing both.

Although the invention has been disclosed in detail in terms of aspecific embodiment, various modifications can be made without departingfrom the scope of the invention. For example, instead of optical fiber,the fiber segment 10 can be made of any material that can be etched asdescribed above, and that can be cleaved to form a (planar) tip. The wetetching can be enhanced by ultrasonic agitation. Instead of isotropicwet etching, other kinds of etching techniques can be used, such as dryplasma etching. The etchings are advantageously, but need not be,isotropic. The two etchings (FIGS. 1-2 and 4) can be chemicallydifferent or physically different (i.e., can be essentially isotropicdry etching in any of the etchings, at some sacrifice of fabricationspeed). The etchings indicated in FIGS. 1-2 and FIG. 4 preferably areboth, but need not be, essentially isotropic.

For use as an NSOM device, the sidewalls of the regions 41 and 42advantageously are coated with an optically reflecting layer such as ametallic layer like chromium, or the fiber segment 10 has a core regionand a cladding region as known in the art (whereby the cladding regionreflects optical radiation during the NSOM use), or both.

In case the fiber segment 10 (FIG. 1) has a cladding as well as a core,advantageously--for use in an AFM device wherein the cladding isoptional, an STM device wherein the cladding is optional, or an NSOMdevice wherein the cladding is described--the diameter of the core (inwhich the chemical composition is uniform) is larger than w (FIG. 4) byan amount in the approximate range of 2.5-to-3.5 μm.

The shape of the cross section of the fiber segment 10 can be other thancircular-such as elliptical, rectangular, or square--as can be obtainedby cutting a glass body into such a shape. In such a case, the crosssection of each cylindrical region has a maximum and a minimum widththat differ from each other.

We claim:
 1. A method of making a probe device comprising the stepsof:(a) providing an initial cylindrical fiber segment; (b) etching,prior to step (c), a lower cylindrical region of the fiber segment for afirst time duration, whereby a width of the lower cylindrical regionbecomes less than that of an upper cylindrical region of the fibersegment; (c) cleaving a tip of the lower region of the fiber, whereby acleaved lower region of the fiber is formed; and (d) etching, subsequentto step (c), the lower cylindrical region of the fiber for a second timeduration, whereby the width of the lower cylindrical region is furtherreduced to a lower value, but the width of the upper cylindrical regionremains essentially unchanged.
 2. The method of claim 1 in which thelower value is in the approximate range of 0.01 μm to 150 μm.
 3. Themethod of claim 1 in which the lower value is in the approximate rangeof 0.05 μm to 0.2 μm.
 4. The method of claim 1 in which the cleaving isperpendicular to the axis of the lower cylindrical region.
 5. A methodin accordance with claim 1 followed by coating the sidewalls of thelower cylindrical region with an optically reflecting layer.
 6. A methodin accordance in claim 1 further comprising, prior to the step (b), thestep of coating the sidewalls of an uppermost cylindrical region of thefiber segment with a protective layer that is not etched by the etchingof the step (b) and removing a lower portion of the protective layerprior to the step (d).
 7. The method of claim 6 in which the protectivelayer is formed by dissolving a chlorofluorocarbon polymer resin in anorganic solvent.
 8. The method of claim 7 in which the initial fibersegment is essentially glass.
 9. The method of claim 6 in which theprotective layer is formed by dissolving a chlorofluorocarbon polymerresin in a ketone or an ester or a mixture of a ketone and an ester. 10.The method of claim 9 in which the initial fiber segment is essentiallyglass.
 11. The method of claim 1 in which the etchings of steps (b) and(d) are essentially isotropic and in which lower value is in theapproximate range of 0.01 μm to 150 μm.
 12. The method of claim 11 inwhich the lower value is in the approximate range of 0.05 μm to 0.2 μm.13. A method including making the probe device in accordance with claim1, 2, 4, 5, 6, 7, 8, 9, 10, 11, or 12, followed by moving the probedevice across a surface of a sample body.